The present invention relates to global navigation satellite systems (GNSS), and more particularly, to methods for processing external correction messages, correcting position measurements of a GNSS receiver, and related apparatuses.
The global navigation satellite systems (GNSS), such as Global Position System (GPS), Galileo, or GLONASS, are widely used in many applications. A GNSS receiver can determine its position by receiving and analyzing coded signals transmitted from a plurality of orbiting satellites. The GNSS receiver computes the difference between the time a satellite transmits its signal and the time that the GNSS receiver receives the signal. The GNSS receiver then calculates its distance, or “pseudo-range,” from the satellite in accordance with the time difference. Using the pseudo-ranges from at least four satellites, the GNSS receiver can determine its three-dimensional position (i.e., latitude, longitude, and altitude).
Unfortunately, the GNSS receiver has potential position errors due primarily to a variety of unintended sources, such as ionosphere and troposphere delays, receiver clock error, satellite orbit drift (a.k.a. ephemeris errors), etc. Most of the errors are “common errors” that are experienced by all the GNSS receivers in a local area.
To improve the accuracy of position measurement of the GNSS receiver, differential global positioning systems (DGPS) were developed. Conventional DGPS uses a stationary GNSS receiver at a known location as a reference station. The reference station measures satellite signal error by comparing its known position with the position measurement derived from the received satellite signals, and then transmits GNSS differential correction information (e.g., timing error measurements) to GNSS receivers within the area covered by the reference station. The GNSS differential correction information is applied to the position calculations of the GNSS receivers so that the GNSS receivers can get a more accurate position measurement.
A well-known example of DGPS is the Radio Technical Commission for Maritime (RTCM) Service provided by the U.S. Coast Guard. Generally, the GNSS receiver can receive GNSS differential correction data carried by the RTCM messages from a beacon, Internet, or through an RS232 cable.
The Satellite Based Augmentation System (SBAS) is another source of GNSS differential correction data. There are several types of SBAS, such as the Wide Area Augmentation System (WAAS) of North America, the Canada-Wide DGPS Correction Service (CDGPS) of Canada, the Multi-Functional Satellite Augmentation System (MSAS) of Japan, and the European Geostationary Navigation Overlay Service (EGNOS) of Europe. The SBAS satellites broadcast SBAS messages containing GNSS differential correction data to GNSS receivers within the coverage area of the SBAS satellites. The GNSS receivers with SBAS capabilities are capable of using the GNSS differential correction data carried by the SBAS messages to correct the GNSS satellite signal errors.
In addition to the RTCM and SBAS, some cellular communication systems (e.g., GSM) can also be utilized as a source of GNSS differential correction data. For example, a GSM base station can directly transmit A-GPS messages containing GNSS differential correction data to GNSS receivers with A-GPS capabilities through a wireless network.
As described previously, there are many sources of GNSS differential correction data. However, the data format and contents are different from each other. If a GNSS receiver wants to support multiple types of the GNSS differential correction data, considerable amounts of memory are required, thereby significantly increasing the hardware cost.